I. Field of the Invention
The present invention relates to medical devices. More particularly, the present invention relates to a minimally invasive surgical procedures.
II. Description of the Related Art
The use of minimally invasive surgical techniques has dramatically affected the methods and outcomes of surgical procedures. Physically cutting through tissue and organs to visually expose surgical sites in conventional "open surgical" procedures causes tremendous blunt trauma and blood loss. Exposure of internal tissues and organs in this manner also dramatically increases the risk of infection. Trauma, blood loss, and infection all combine to extend recovery times, increase the rate of complications, and require a more intensive care and monitoring regiment. The result of such open surgical procedures is more pain and suffering, higher procedural costs, and greater risk of adverse outcomes.
In contrast, minimally invasive surgical procedures cause little blunt trauma or blood loss and minimize the risk of infection by maintaining the body's natural barriers to infection. Minimally invasive surgical procedures result in faster recoveries and cause fewer complications than conventional surgical procedures. Minimally invasive procedures, such as laparoscopic, endoscopic, or cystoscopic surgery, have replaced more invasive surgical procedures in all areas of medicine. Due to technological advancements in areas such as fiber optics, micro-tool fabrication, noninvasive visualization, and material science, the physician performing the operation has easier-to-operate and more cost effective tools for use in minimally invasive procedures. However, there still exist a host of technical hurdles that limit the efficacy and increase the difficulty of minimally invasive procedures.
One critical aspect of minimally invasive surgical techniques is the ability of the operator to visualize of the position of surgical instruments within the body and to determine the extent of the manipulation of organs and tissues caused by the surgical instruments. For example, percutaneous coronary angioplasty (PTCA) uses fluoroscopy to position a tiny balloon at the end of a long flexible catheter in a coronary artery where a stricture has reduced blood flow to the heart. Expanding the balloon at the site of the stricture opens the blocked arteries and normal blood flow returns. Visualization of the position of the balloon and the extent of tissue manipulation by the balloon is accomplished by injecting radiographic fluid through the catheter into the arteries. Thus, fluorscopy provides a detailed picture of the boundaries of the vessels to be treated, the position of the balloon within the vessel, and the extent and result of tissue manipulation by the balloon. Fluoroscopy is a critical aspect of the PTCA procedure because it provides the operator with a method for determining the exact position of the PTCA balloon in the artery and the effectiveness of the balloon in opening the stricture.
Another example of a minimally invasive procedure is arthroscopic surgery in which fiber optic visualization is used to position and control small tools within bone joints to repair, ablate, or remove tissue. Instruments, such as fiber optic scopes, small surgical tools and aspiration tubes, are placed in the joint through small skin incisions. The entire operation is performed by the operator visualizing the affected area through the fiber optic scope. Direct visualization is effective because the operator can expand the joint with a clear fluid and illuminate the surgical field.
Fiber optic and fluoroscopic visualization require a vessel, duct, or cavity into which a clear or radiopaque fluid can be injected. However, procedures not involving a vessel, duct, or cavity require other methods of visualization. For example, the manipulation of soft tissue organs requires entirely different methods of visualization. In such procedures, visualization techniques such as magnetic resonance imaging (MRI) or ultrasound which distinguish the borders and shapes of soft tissue organs or masses are required.
MRI has been particularly effective in providing detailed visualization of damage or growth of soft tissues surrounded by other organs and structures. MRI measures the radio frequency (RF) signals emitted by the nuclei of atoms subjected to a transient magnetic field while in a strong static field. These measurements define the nature and orientation of the atoms to provide a detailed image of the boundaries, geometry, and composition of tissues, organs or structures within the body. MRI's are routinely used to visualize herniated discs in the spinal cord, brain tumors, cancerous lesions deep within the body, and soft tissue damage to tendons and ligaments.
One prior art use for MRI is the visualization of the prostate and the progression of the growth of the iceball or frozen tissue surrounding a cryosurgical probe during the cryoablation of the prostate. MRI can be used to optimize probe placement and cooling parameters, to monitor the temperature distribution within the frozen region, and to determine the extent of tissue damage after the procedure. For an example of a procedure in this field, see U.S. Pat. No. 5,433,717 to Rubinsky et al. However, the size, cost, complexity, and nature of MRI systems may make them poor candidates for visualization in many surgical procedures.
Ultrasound imaging relies on the reflection of high frequency sound waves at interfaces of varying acoustic impedance to create a two dimensional picture of internal body structures. Present ultrasound imaging systems provide little detail and are limited by the sound transmission properties of obstructions in the field of view. Ultrasound provides a lower cost, less complicated and more compact alternative to MRI. Ultrasound imaging systems can typically be operated by one person, are a fraction of the cost and size of an MRI system, are mobile, and are less restrictive on the operating environnent.
The use of ultrasound in medicine is well documented and a considerable volume of patent literature exists describing designs, methods, and instrumentation in the area of ultrasound imaging and the use of ultrasound as a diagnostic or therapeutic tool. One application of ultrasound has been the evaluation of the structure and density of bone. For example, ultrasonic calipers containing an ultrasonic emitter and receiver have been used to measure the amplitude and velocity of an ultrasonic signal transmitted through bone tissue, such as the bone in a finger. The amplitude of the ultrasonic signal depends on the absorption of the ultrasound passing through the bone tissue. Other systems utilize a pair of ultrasound transducers, one emitting and the other receiving a composite sine wave acoustic signal consisting of repetitions of plural discrete ultrasonic frequencies spaced at discrete times. The signal is processed to determine a transfer function. The transfer function is then used to allow the evaluation of bone-related quantities such as bone density, bone strength, and fracture risk. For examples of patents in this area, see U.S. Pat. No. 5,564,423 to Mele et al. and U.S. Pat. No. 5,259,384 to Kaufman et al.
Modern ultrasound technology has expanded the application of minimally invasive surgical techniques into areas in which direct surgical intervention had previously been the only option. One surgical technique to which modern ultrasound imaging has been applied is cryosurgery. Cryosurgery involves the freezing of diseased tissue. Cryosurgery has been used for decades to destroy diseased tissue throughout the body. Historically, cryosurgery has been limited in its application to the destruction of tissue on the surface of the body or in a space where the visible manipulation of tissue was possible. Recently the role of cryosurgery has been expanded to include the application of cryosurgery in a minimally invasive manner. Minimally invasive techniques were made possible by the advances in the ability to visualize soft tissues by ultrasound imaging. Ultrasound imaging allows the surgeon to visualize "landmarks" within the patient and, thereby, correctly position the freezing probe or cryoprobe within the soft tissue.
Both heating and freezing of tissue affects its acoustic properties. In the case of heating and freezing tissue, the transmission of ultrasound signals through the tissue is affected by the change in density of water within the tissue. Although small, the change in density of water as a function of temperature is well documented. As an example, the density of water at 38 degrees Celsius is 0.99299 gm/ml and at 70 degrees Celsius is 0.97781 gm/ml. In the case of freezing, the density of ice is 0.9170 gm/ml. These density changes affect the attenuation of the ultrasound signal transmitted between the imaging system scanner and the probe transducer during the manipulation process.
An example of the application of cryosurgery in a minimally invasive manner is the destruction of diseased prostatic tissue using small diameter cryoprobes placed through the skin into the prostate. Ultrasound imaging allows the surgeon to view the prostatic margins, bladder, urethra, and rectum. Using these organs as "landmarks," the surgeon positions the cryoprobe within the prostate to optimally destroy the diseased tissue. Information concerning an ultrasound transducer which is attached to a cryoprobe can be found in U.S. patent application No. 08/468,717, filed Jun., 6, 1995, now issued as U.S. Pat. No. 5,672,172, entitled "SURGICAL INSTRUMENT WITH ULTRASOUND PULSE GENERATOR" which is assigned to the assignee of the present invention and which is incorporated herein by this reference. The patent describes a system and method which further enhances the ability of the surgeon to position the cryoprobe to optimally destroy diseased tissue by clearly marking the position of the tip of the cryoprobe in the display of an ultrasound imaging system.
Ultrasound imaging also provides the surgeon with a method for judging the extent of the manipulation by the cryoprobe. After positioning the cryoprobe within the desired tissue mass, the liquid nitrogen begins to flow to the cryoprobe. Tissue begins to freeze around the tip of the cryoprobe. As the flow of liquid nitrogen continues, thermal energy is removed from an expanding area of tissue surrounding the tip of the cryoprobe. The temperature profile of tissue manipulated by the cryoprobe is lowest at the tip and increases to the boundary of manipulated and unaffected tissue at normal body temperature. It is generally accepted that tissue destruction occurs at temperatures below minus 60 degrees Celsius, although such factors as the speed of freezing and thawing of the tissue and the water content of the tissue affect the temperature at which tissue destruction occurs.
Within this region of manipulated tissue is a boundary where the tissue temperature falls below 0 degrees Celsius. This area of frozen tissue is described as the "iceball." The boundary of the iceball is echogenic as a result of the difference in density between frozen and unfrozen tissue. As thermal energy is removed from the tissue and the iceball grows, the boundary appears as an expanding arc in the display of an ultrasound imaging system. Monitoring the growth and position of the arc gives the surgeon an indication of the extent of the manipulation of the tissue by the cryoprobe. However, as noted above, the actual destruction of tissue does not occur until the tissue reaches approximately minus 60 degrees Celsius. Therefore, the echogenic boundary is not directly indicative of the extent of tissue destruction. In addition, the ultrasound image provides the surgeon with no definite indication of the nature of the tissue within the iceball.
An alternative to the above description of a manipulation process employing cryosurgery is a manipulative process involving heating of tissue. In such a case, the manipulation tool is not a cryoprobe, but rather a probe to deliver microwave, RF, or other energy form that heats tissue. Such devices are commercially available and are presently used to ablate tissue of the prostate and endometrium. Heating tissue causes cell destruction leading to necrosis of tissue within a thermal zone surrounding the heating component. Just as in cryosurgery, monitoring of the effected zone is critical during the heating process to ensure destruction of all diseased tissue and the preservation of healthy surrounding tissue.
The surgical procedure for destroying a target tissue with a heating probe begins by placing the probe in the target tissue mass. Ultrasound imaging is typically used to guide the probe in a minimally invasive manner. An ultrasound transducer on the probe in close proximity to the heating element is used to facilitate the positioning of the probe. In similar fashion to the freezing probe ultrasound transducer, the heating probe ultrasound transducer generates an ultrasound signal. The signal is detected by the imaging system causing an indication to be displayed in the monitor of the imaging system at the ultrasound transducer on the probe. Thus, the imaging system defines the exact location of the probe tip and heating element. As the manipulation process heats tissue surrounding the probe tip, the ultrasound transducer continues to produce an image of the probe and surrounding tissue. Adding thermal energy to tissue changes the structure and density of the tissue, and, thus, affects the image seen by the surgeon. It is generally accepted that tissue can not tolerate temperatures greater than 60 degrees Celsius. However, the image created by an ultrasound scanner which is seen by the surgeon does not provide a definite indication of the boundary of the tissue which has reach 60 degrees Celsius. Thus, the surgeon is not provided with a good indication of the extent of tissue manipulation.
Therefore, it is apparent that there has been a long felt need in the industry to accurately determine the extent of tissue manipulation as a function of time. The present invention provides an elegant means and method for satisfying this need.